It is 120-theta because those are the only other angles that fill the 180 degrees of angles under the eyebolt (referring to quadrants 3 and 4 making up half the circle or 180 degrees). Then he took 180-theta-alpha. So when the law of sines asks for the sine of that angle alpha, he can just substitute the 120-theta to solve for the direction angle.

CrazyCloud9613

why is it 120 - theta?

AbOoOD ZERO

the angle should be 76.5 clockwise from x-axis , that's mean below x-axis